1. Field of the Invention
The invention relates to a closed-loop control device for a process in which a dead band element is connected upstream of a linear dynamic controller.
2. Description of the Related Art
The function manual “Prozessleitsystem PCS 7, PCS 7 Advanced Process Library V71”, March/2009, discloses PID controllers, which may also be referred to as P, PI or PD controllers depending on the configuration, or model-predictive controllers, which are referred to as ModPreCon there, for example, as linear dynamic controllers that can have a dead band element connected upstream of them when used in a closed-loop control device. The transfer function of a dead band element described there is illustrated in FIG. 2. With a dead band element connected upstream, a control difference that is passed to the controller is formed from an effective desired value SP and a process value PV and is available to the controller at the output ER. In order to suppress disturbances in the steady state or to suppress unavoidable measurement noise, the dead band can be activated and the lower band limit −D (dead band) can be parameterized together with the upper band limit D. If D is set to 0, then the dead band is deactivated but, if D is not equal to 0, then the dead band is activated. The dead band is parameterized by the user and is constant during operation. Here, the dead band is ideally parameterized such that the likelihood of actuating interventions is low if the controlled variable has stabilized inside the dead band. This requires a sensible selection of the width of the dead band.
The width of the dead band primarily depends on the control precision desired by the process, i.e., on the maximum permissible remaining control deviations and possibly, for example, if a switching actuator is used, on the change in the controlled variable with the minimum possible change in the manipulated variable. With regard to minimizing actuating interventions, the following setting rules help in this case to select the dead band to be so wide that variance in the controlled variable, which is unavoidable on account of measurement noise or quantization noise for example, does not result in frequent actuating movements.
Firstly, assuming a normal statistical distribution of the values of the controlled variable around the desired value, two to three times the standard deviation of the controlled variable in the steady state is used as the width of the dead band. If the closed-loop control device is provided with a monitoring module for determining characteristic variables for the control behavior, which is also referred to as control performance management (CPM) or ConPerMon in the function manual mentioned at the outset, the standard deviation of the actual value, calculated in the monitoring module, can be used to calculate the dead band width.
Secondly, if the controlled variable is changed only gradually by quantizing the manipulated variable, for example, on account of pulse width modulation with a defined minimum pulse duration, on account of a step controller with a defined minimum step size or in the case of an electropneumatic position controller with static friction in the stuffing box of the pneumatic drive, then the width of the dead band depends on the manipulated variable quantization that should be multiplied by the respective process gain. For example, in the case of temperature control with manipulated variable quantification of 5% and a process gain of 1.5° C./%, only temperatures in a grid of5%·1.5° C./%=7.5° C.can be accurately achieved. The dead band must then be selected to be so wide that at least one grid point, at which the process can stay in a steady state, falls in the dead band. The limit D of the dead band should be set to be greater than approximately 3.8° C. in this example.
Although a certain reduction in the actuating interventions is already achieved with a dead band of constant width, some disadvantages must be accepted.
Firstly, control deviations that are smaller than the dead band are ignored by the controller, i.e., the controller does not correct these deviations even if the controller could do so without an upstream dead band. Therefore, steady states of the actual value of the controlled variable may be established, the temporal mean value of which deviates considerably from the desired value.
Secondly, if steady states are established in the edge regions of the dead band, then a control intervention may result again and again even with the slightest disturbances. This is because, after a disturbance event that results in the dead band being left, the controller returns the actual value of the controlled variable only to the respective edge of the dead band, which may result in new actuating interventions in the event of further disturbances. This increases the wear and energy consumption of the actuator.
Thirdly, large dead bands particularly have an adverse effect on the control behavior of the control loop in the event of sudden desired value changes. The reason for this is that the controller first of all “stops work”, for example, in the rise phase of a positive step response when the control difference enters the dead band. This may result in a creeping stabilization process or in the controller only becoming active again when the actual value exceeds the upper limit of the dead band on account of an overshoot.
A dead band element connected upstream of the controller in a closed-loop control device can thus contribute, as is known, to reducing actuator movements and thus the energy consumption and the wear of the actuator. This applies, in particular, to mechanical actuators, such as valves and pumps. Furthermore, a dead band element is able to avoid sustained oscillations as an operating movement of an actuator, which are caused by the quantization of the actuating interventions, in the control loop.
However, use of an upstream dead band element has hitherto usually resulted in impairment of the control quality because the exact desired value is achieved only with an offset dependent on the dead band width and not by the mean value of the controlled variable, as when the dead band element is omitted. The offset of the controlled variable with respect to the exact desired value also results in the dead band actually selected for the purpose of reducing actuating interventions being left more frequently because the dead band is symmetrical around the desired value and not around the achieved actual value of the controlled variable. Upon leaving the dead band, the controller must again intervene to return the actual value to the region of the dead band again, with the result that the abovementioned advantages of a dead band can be achieved only to a lesser extent.
These problems result in dead band elements often being connected upstream of linear dynamic controllers only in those cases in which they are absolutely necessary and usually not in cases in which only the wear and energy consumption of a process system, in which the closed-loop control device is used, are intended to be reduced.